With the exception of crude distillation, reforming is probably practiced more widely throughout the world than any other hydrocarbon processing reaction. In reforming, naphthene rings derived from paraffins are dehydrogenated into aromatic rings in the presence of a catalyst. The reformate will usually contain from 35 to 60 percent by weight of benzene, toluene and xylenes. Reforming catalysts are usually noble metals, such as platinum, or mixtures of platinum metals such as platinum and rhenium, on acidic supports such as alumina. Potential problems indigenous to reforming include polynuclear aromatic content in the reformate and heat balance in the overall endothermic catalytic process.
A process for the suppression of catalyst deactivation and for maintaining proper heat balance was disclosed in Clem et al, U.S. Pat. No. 4,125,454, where a series of reactors for reforming is positioned where the reactor next scheduled to have its catalyst-regenerated is located immediately downstream of a reactor which contains freshly regenerated catalyst. In this manner, any sulfur expelled during reforming in the presence of newly regenerated catalyst is adsorbed on the coke of the catalyst which is in a near spent state. In Bonacci et al, U.S. Pat. No. 4,292,167, a process is disclosed for a multi-stage reforming reactor where the first reforming stage effluent is cooled and contacted with a ZSM-5 type zeolite prior to intermediate heating between the reformer zones. It is recognized that as naphtha passes through each reforming stage, the endothermic reactions which take place result in a lowering of the temperature. This requires intermediate heating between the reformer stages. It is recognized that reforming catalyst is not particularly active at temperatures below 800.degree. F. and therefore each reactor should be designed to operate at exit temperatures above this particular level. Many sophisticated systems have been developed to heat balance multi-stage reforming reactions. One such sophisticated heat exchange method is disclosed in Scott, U.S. Pat. No. 3,981,792.
In Hofer et al, U.S. Pat. No. 3,689,404, an adsorption process is described using activated carbon to remove naphthalenes and alkyl substituted naphthalenes from refined petroleum fractions including catalytic reformate. In this manner dicyclic hydrocarbons are separated from a mixture of dicyclic, monocyclic and paraffinic hydrocarbons utilizing an activated carbon adsorbent. In Wackher et al, U.S. Pat. No. 3,340,316, a process is described for the separation of aromatics by means of activated carbon containing a polar fluoride molecule and having as a cation ammonia or an element from Groups I, II and III of the Periodic Table. This separation may be performed on charge stocks such as those derived from thermal or catalytically cracked materials, catalytically dehydrogenated petroleum fractions and straight run distillate fractions. It is recognized that processes for the separation of aromatic hydrocarbons using selective sorbents are well known in the art and that U.S. Pat. No. 2,819,326, Mills, is disclosive of a process for the separation of polynuclear aromatics from mononuclear aromatics employing silica gel as the selective sorbent. Certain coke precursors to thermal or catalytic cracking reactors are removed from motor fuel range hydrocarbons by means of silica gel or alumina (U.S. Pat. No. 2,632,727, Lanneau et al). By this method all potential coking poisons are removed from the feedstock by adsorption on silica gel prior to cracking.
Separation of aromatics from gasoline reformates, especially catalytic reformates with an adsorbent such as activated charcoal or activated alumina, is disclosed in Shuman, U.S Pat. No. 2,867,582. Recognition of the problems of polynuclear aromatics in motor fuel reformates was disclosed in Hudson et al, U.S. Pat. No. 4,608,153. However, in deference to removing these polynuclear aromatics by means of adsorption, a catalytic system was devised containing elemental iron, one or more alkali metal or alkaline earth metals of the Periodic Table and a Group III-A compound such as alumina. Certain crystalline dehydrated zeolites have been shown as molecular sieves for an adsorption process to separate paraffins from aromatic materials. See Henke et al, U.S. Pat. No. 2,940,926. The subject of such separation is taught as a catalytic reforming product to enable recycle of the saturated fraction to catalytic reforming while recovering high octane aromatic hydrocarbons. Graphite has also been shown as a material for hydrocarbon separation as taught in Geach et al, U.S. Pat. No. 3,531,089. Finally, a multi-stage adsorption separation technique for hydrocarbons is disclosed in Woodle, U.S. Pat. No. 3,767,563. Other separatory processes include U.S. Pat. Nos. 4,032,431; Weisz, 4,447,315; Lamb, and 4,411,768 Unger.
These disclosures have failed to recognize the harmful cumulative effect of the polynuclear aromatics as the reforming hydrocarbons pass through the adiabatic reforming stages. There is no recognition in this art of the tremendous reduction achieved in the reformate polynuclear aromatic content by intermittently eliminating the polynuclear aromatics from the beginning of each reforming bed. There is also a failure to recognize that as a hydrocarbon is passed through each respective reforming bed that polynuclear aromatic content accumulates exponentially during endothermic reforming.